1. Field of the Invention
The present invention relates to an organic light-emitting display, and more particularly, to an organic light-emitting display which minimizes color shift, increases brightness and improves visibility.
2. Description of Related Art
In general, an organic light-emitting diode (OLED) includes an anode, an organic light-emitting layer and a cathode. When a voltage is applied between the anode and the cathode, holes are injected from the anode into the organic light-emitting layer, and electrons are injected from the cathode into the organic light-emitting layer. Holes and electrons that are injected into the organic light-emitting layer recombine with each other in the organic light-emitting layer, thereby generating excitons. When such excitons transit from the excited state to the ground state, light is emitted.
Organic light-emitting displays including an OLED are divided into a passive matrix type and an active matrix type depending on a mechanism that drives an N×M number of pixels which are arranged in the shape of a matrix.
In the active matrix type, a pixel electrode which defines a light-emitting area and a unit pixel driving circuit which applies a current or voltage to the pixel electrode are positioned in a unit pixel area. The unit pixel driving circuit has at least one thin-film transistor (TFT). Due to the TFT, the unit pixel driving circuit can supply a constant current irrespective of the number of pixels, thereby realizing uniform brightness. The active matrix type organic light-emitting display consumes little power, and thus can be advantageously applied to high definition displays and large displays.
However, the organic light-emitting display has a low out-coupling efficiency, which is problematic. For example, in the case of an organic light-emitting display which has not undergone separate processing, about 20% of light emitted from the organic light-emitting layer exits the display.
Here, the light efficiency of the organic light-emitting display is determined by the refractive indices of individual component layers of the organic light-emitting display, including the organic light-emitting layer to the outer portion thereof. One of the factors that reduce the light efficiency is that a portion of light travels in an unnecessary direction when exiting from a substrate having a higher refractive index to the air having a lower refractive index and, when incident on the interface between the substrate and the air at an angle greater than a critical angle, totally reflects from the interface, thereby reducing the amount of light that exits to the outside.
In order to overcome the light efficiency problem of the organic light-emitting display, a micro cavity structure was proposed. In the micro cavity structure, the distance between the anode and the cathode is designed to match the representative wavelength of each of red (R), green (G) and blue (B) light such that only the light corresponding to the distance resonates and exits but the other light is weakened. Consequently, the exiting light is increased in intensity and becomes sharper, so that its brightness is advantageously increased. The increased brightness results in low power consumption, which leads to increased longevity. Here, the fact that the emitting light becomes sharper indicates that color reproducibility is increased with improved color purity.
Although the organic light-emitting display having the micro cavity structure exhibits the foregoing advantages, it also has the drawback of a limited color viewing angle that is caused by color shift. This is because the optical path is changed at a side, or a high angle, thereby varying the wavelength of light that can resonate. Therefore, since the optical path is increased at the side, the light that resonates and exits is more shifted toward a short wavelength, which is problematic.
Furthermore, since the organic light-emitting display uses the OLED that is a self-luminous light source, the anode is made of a high-reflectivity material in order to effectively send emitted light only toward the front. Consequently, in external light conditions, the visibility of the organic light-emitting display is decreased due to very high reflectivity.
The information disclosed in the Background of the Invention section is only for the enhancement of understanding of the background of the invention, and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.